This invention is concerned with a techniqu determining the liquid water content of an ambient aerosol.
An ambient aerosol is composed of a variety of chemical components which influence the behavior of the aerosol in the atmosphere. Some species of these components are chemically reactive with other gas phase components, resulting in changes in aerosol size and chemical composition over time. Other compounds found in ambient aerosols, such as hygroscopic inorganic salts (sodium chloride and ammonium sulfate, for example), are chemically stable but may cause changes in size and composition due to their tendency to adsorb water vapor from the surrounding air. These size changes can become significant at high (greater than 70%) relative humidities, resulting in increased light scattering and reduced visibility. In order to adequately assess the importance of liquid water to aerosol chemistry, it is necessary to determine the amount of free (chemically unbound) water in ambient aerosols. To date, no instrument available in the art has been capable of making these measurements over a broad range of ambient temperatures and relative humidities.
A direct, quantitative measurement of the liquid water content of an aerosol is very difficult to make, because under some conditions this value may be very small (less than 10 ug/m.sup.3 (micrograms per cubic meter), for example) with a much larger amount of water vapor contained in the surrounding air (19 g/m.sup.3 (grams per cubic meter), for example, for saturated air at 70.degree.). Ideally, the measurement of liquid water content should be made in situ, using an instrument which can differentiate liquid water from water vapor with a precision approaching 0 ug/m.sup.3. Several attempts have been made to determine aerosol liquid water content under these conditions. None of the instruments in the prior art, however, has proven capable of accurately making these measurements over a realistic range of ambient temperatures, relative humidities, and aerosol concentrations.